Composite rotor blades conventionally are mechanically retained to a rotor hub by a root fitting which effectively transfers dynamic loading from the composite blade structure, through the fitting, to the rotor hub. In rotor blades of advanced composite structure, problems arise in transmitting the dynamic loads to the rotor hub, including the torque, tension and bending loads on the rotor blade which must be transmitted to the rotor hub. In other words, the attachment means provides a path for loads from the composite structure to the rotor hub.
In the case of metallic blades which are retained for rotation upon the periphery of the rotor hub, a wide variety of mechanical fittings or attachment means are available and are quite effective. However, the trend toward incorporating composite blades into rotor assemblies has produced unique problems not heretofore experienced. By the term "composite blades" it is meant those blades formed by laminating multiple plies of elongated, small diameter filaments of high strength, i.e. high modulus of elasticity, embedded in a lightweight matrix. Typical examples are the non-metallic composites such as graphite filaments in an epoxy resin, and certain metallic composites in a matrix such as aluminum.
The interface of composite materials in rotor blades to metallic elements of the rotor hub has been analytically and experimentally shown to be the crucial link in making composite rotor blade assemblies. This is the area where most problems occur, i.e. the area of transmitting the dynamic loads on the composite rotor blade to the metallic rotor hub. In the past, spade-type configurations have been used for the composite-to-metallic interface. Fibers from the composite blade simply have been wound around the spade and bonded thereto with a resin. However, during fatigue life testing, the resin bonded to the metallic spade tends to separate and fail at that interface.
A variety of attempts have been made to solve the above problems, some of which are shown in U.S. Pat. Nos. 3,603,701 to Tarcrynski, dated Sept. 7, 1971; 3,731,360 to Stone, Jr., dated May 8, 1973; and 4,031,601 to Staub, dated June 28, 1977. Tarcrynski shows a system for transferring the dynamic loading forces from a composite rotor blade held between opposing external and internal metallic fittings, with a lock nut in communication with a key to transmit torque from the internal fitting by a preloading scheme. Stone shows a method of making a composite blade with an integral root, in a compacting and bonding operation, and positioning a wedge between spread layers of the composite blade structure at the root end thereof. Staub shows a rather complicated dovetail slot interface between a fiberglass blade and a hub, including forcing additional, substantially liquified resin and fiberglass material through a bore in the hub fitting.
This invention is directed to providing a new and improved system for retaining a rotor blade on a rotor hub in which a gripping action occurs and the retention forces actually increase in response to dynamic loading forces on the blade.